1. Field of the Invention
The present invention relates to an improvement of a cathode structure of a transverse excitation type laser oscillator.
2. Description of the Prior Art
A CO.sub.2 laser oscillator as a typical transverse excitation type laser oscillators will be described in detail.
In the transverse excitation type CO.sub.2 laser oscillator, the injection discharge power to a laser medium per unit volume can be increased by using a highly pressurized laser medium and a cooling of the laser medium can be effectively performed by forcible circulation whereby a remarkably high output can be obtained in comparison with the conventional glass tube type CO.sub.2 laser oscillator.
In this CO.sub.2 laser oscillator, it is necessary to maintain the stable discharge in space uniformity, in high pressure and high speed gas flow. It is not easy to maintain the stable glow discharge when the pressure is high.
FIGS. 1(a), (1b) are sectional views of the important parts for illustrating the glow discharge of the usual transverse excitation type CO.sub.2 laser oscillator and show one plane anode and the nearest one of rod cathodes which are arranged to face the anode.
In FIG. 1(a), the rod cathode is arranged in substantially perpendicular to the anode. In FIG. 1(b), the rod cathode is arranged in slant to the downstream of the gas flow.
In FIGS. 1(a), (1b), the reference numeral (1) designates a plane anode; (2) designates a rod cathode; (3) designates a support insulating substrate; (4) a stabilizing resistor; (5) designates a DC power source; and (6), (61), (62) designate glow discharge regions. The normal glow discharge (61) is sometimes shifted to (62). The arrow line Ga shows a flow direction of the laser medium. A laser resonator, which is not shown in the Figures, has its optical axis in the direction perpendicular to the drawing and comprises a pair of reflectors arranged to face each other.
In the laser oscillator having the above-mentioned structure, a mixed gas of CO.sub.2, N.sub.2 and He is used as the laser medium and the glow discharges are observed at a pressure of the laser medium of 0.1-1 atm. and a flow rate of 10-90 m/s. As result, in the structure of FIG. 1(a), the cathode spot of the glow discharge is vertically shifted on the surface of the cathode (2) in the downstream whereby the glow discharge region is not stable and is shifted between (61) and (62). The instability of the discharge region is increased depending upon increasing the pressure of the laser medium. The instability of the discharge region is serious trouble to increase the output of the laser oscillator.
The structure of FIG. 1(b), is considered to prevent such trouble. The cathode (2) is arranged in slan in the downstream of the gas flow, whereby the glow discharge region (6) is maintained at the top of the cathode (2) to prevent said trouble.
However, the following trouble is caused when a laser oscillator having greater output is considered even though said structure is given.
In the case of such cathode (2), it is necessary to increase field intensity near the cathode (2) in order to maintain the stable glow discharge touching the cathode glow on its top. Accordingly, it has been proposed to form a sharp top of the cathode (2) or to process a top of a fine wire having a diameter of about 1 mm so as to form a round top.
However, even though the cathode is processed in such shape, the glow discharge region (6) can be stable under maintaining the shower form as shown in FIG. 1(b) at only lower than about 10 mA of the discharge current per rod of the cathode in the case of about 1 atm. of the pressure of the laser medium. When the discharge current is larger than about 10 mA, a pulse discharge in a filamental form is intermittently formed. Such phenomenon is found in a step converting from the glow discharge to an arc discharge. When the discharge current is increased over said value, the arc discharge is formed whereby a laser output is not obtained. Accordingly, the discharge current per rod cathode is limited to about 10 mA. In order to increase the output of the laser oscillator, it is necessary to increase the number of the cathodes and to require larger size of the laser oscillator. This is a serious trouble for constructing a laser oscillator having large output.